I had the same problem. But my phone would also not ring. I was on the phone with Apple Care for a couple of hours and nothing helped at first. We restored, restarted and what seemed like a bazillion other things. We finally got it back by making a phone call and pressing the speaker phone button then turning the speaker phone off again. But it happened again while I was on the phone and we did the same fix. Finally he said bring it back to an Apple store, which I did. Apple ran a couple of diagnostics and could not find anything. They exchanged it, no problem. Still don't know what the problem was.

Happened to my first 4 and the "geniuses" told me to restore the phone and set it up as a new device. I tried that and it still lost it's sounds every once and a while. Not sure what the problem was but they reluctantly swapped the phone out and it hasn't done it since. Might go talk to them?

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